feat: Add to_json and from_json functions

proposals/0019-json-str-functions.md

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+# Title of the feature
+
+- JEP: (leave blank)
+- Author: tmccombs
+- Created: 2023-05-30
+
+## Abstract
+[abstract]: #abstract
+
+Add two new functions which allow parsing a string into a JSON value, and serializing a value to a JSON string.
+
+## Motivation
+[motivation]: #motivation
+
+JSON values sometimes contain strings, which are themselves JSON. As a specific example, some AWS APIs
+return responses that include values which are strings containing JSON strings, such as IAM policy
+documents.
+
+Previously, it was not possible to extract data from such a string directly with JMESPath. At least without
+having to use one pass to extract the string value, then use a different expression the result of parsing the
+JSON from that string.
+
+Conversely, if you need to serialize data into a string field of the result, then if your input is not a string
+you can currently use `to_string`, but that function won't correctly serialize an existing string as a JSON string.
+If you don't know the actual type of the value, and it could be a string, you can't safely serialize as a JSON string.
+
+## Specification
+[specification]: #specification
+
+This JEP defines two new functions `from_json` and `to_json`.
+
+### `from_json`
+
+The `from_json` function has the following signature:
+
+```
+any from_json(string $json_data)
+```
+
+This function takes a string, and will parse it as a JSON document and return the resulting value. The resulting
+value may be any JSON type.
+
+If the input string is not a valid JSON document, an `invalid-value` error is raised.
+
+### `to_json`
+
+The `to_json` function has the following signature:
+
+```
+string to_json(any $value)
+```
+
+This function performs the inverse of `from_json`. It takes any JSON value as input, and serialize the result
+as a JSON document, and then returns a string containing that serialized result. The resulting JSON should not
+contain any unneeded whitespace, including newlines, spaces, or tabs.
+
+## Rationale
+[rationale]: #rationale
+
+The names `from_json` and `to_json` were chosen because the names are symmetric with each other and are similar to
+the names of the corresponding functions in [`jq`](https://jqlang.github.io/jq/manual/#Convertto/fromJSON).
+
+However, other names could also be accptable, such as:
+* `parse_json`/`stringify_json`
+* `json_parse`/`json_stringify`
+* `deserialize`/`serialize`
+
+Another alternative is to add the `from_json` function but not the `to_json` function, since the existing
+`to_string` function can already serialize most values as json. However, the fact that it can't serialize a string as to
+JSON is an unfortunate gap, and it seems odd not to have symmetric functions going in both directions. If only a deserizalization
+function is added, `from_string` may also be an appropriate name.
+
+This specification only supports serializing as minimal JSON without whitespace. This is conjectured to be what is desired in most
+cases where the `to_json` function is used, in order to keep the size of the resulting data small. This is also consistent
+with the existing `to_string` function. However, a future extension could
+provide functionality to output JSON in a "pretty" format including newlines and indentation, either as an additional function, or with
+an optional argument to the `to_json` function. However, this was intentionally left out of scope for this proposal to keep the scope small.
+
+For handlinge invalid JSON input, another possibility is that `from_json` could return some kind of value. However, it would be difficult to distinguish
+between parsing a legitimate value, and inability to parse the string. And in most use cases, invalid JSON is probably an unexpected scenario that should
+be treated as an error.
+
+## Testcases
+[testcases]: #testcases
+
+```yaml
+given:
+  string: "abc"
+  empty_list: []
+  empty_hash: {}
+  object:
+    foo: "bar"
+    bar: "baz"
+    'null': null
+  json: |
+      {
+          "str": "string",
+          "num": 1.25e4,
+          "arr": [1,2, "a"],
+          "nothing": null
+      }
+cases:
+  - expression: from_json(json)
+    result:
+      str: "string"
+      num: 1.25e4
+      arr: [1,2, "a"]
+      nothing: null
+  - expression: from_json(json).str
+    result: "string"
+  - expression: "from_json('true')"
+    result: true
+  - expression: "from_json('false')"
+    result: false
+  - expression: "from_json('123')"
+    result: 123
+  - expression: "from_json('1.5')"
+    result: 1.5
+  - expression: "from_json('1e8')"
+    result: 1e8
+  - expression: "from_json('5e-3')"
+    result: .005
+  - expression: "from_json('\"abc\"')"
+    result: "abc"
+  - expression: "from_json('[]'"
+    result: []
+  - expression: "from_json('{}')"
+    result: {}
+  - expression: "from_json('null')"
+    result: null
+  - expression: "from_json('abc')"
+    error: invalid-value
+  - expression: to_json(object)
+    result: '{"foo":"bar","bar":"baz"}'
+  - expression: to_json(string)
+    result: '"abc"'
+  - expression: to_json(empty_list)
+    result: []
+  - expression: to_json(empty_hash)
+    result: {}
+```